U.S. Pat. No. 4,141,735 discloses that heat treatment of a polyester film at a temperature of from 50.degree. C. to its glass transition temperature (hereinafter occasionally abbreviated as "Tg") tends to prevent the film from generating core-set curl. However, when such a procedure is applied to the formation of the film on an industrial scale (that is, 50 cm or more in width and 500 m or more in length), tightly cinching (force to tighten a roll toward the central axis of the roll) occurs due to heat shrinkage of the film, which is generated during the heat treatment, resulting in deteriorating flatness of the film. This has been an extremely serious problem for photographic photosensitive films (hereinafter occasionally abbreviated as "photographic materials", "photographic films" or "photosensitive films") which are required to have high flatness.
For solving the above described problem, a method of adjusting a creak value between the outer surface and the inner surface of the film to within 10 to 400 g is known, as described in JP-A-6-123937 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). Although this method is sufficiently effective for the film 100 meters or more apart from a winding core in terms of the film length, it is still insufficient for the film closer to the core, that is, less than 100 meters apart from the core in terms of the film length, resulting in a flatness defect. This is attributable to the fact that the stress due to the tightly cinching produced becomes strongest in the proximate portion to the core, eventually leading to a winding distortion defect (a defect of generating twist wrinkles on the film in the width direction like wrung cloth). As a result, this method is disadvantageous in that it cannot sufficiently raise the yield after heat treatment.